Well cementing slurries containing fibers

ABSTRACT

The problem of fiber or particle settling in well cement slurries is addressed by providing a fluid containing two fiber components of differing properties. Such a well treatment fluid, comprising a base fluid; a first fiber component that it substantially more dense than the base fluid; and a second fiber component that has a density close to that of the base fluid and is relatively flexible.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to cementing slurries containing particlesand fibres. In particular, it relates to such fluids in which thesedimentation of the particles or fibres in the slurry is prevented orhindered. Such slurries find applications in well cementing operations.

2. Description of Related Art

The use of particles or fibres in well treatment fluids such as cementshas been previously proposed. One such example is described in EuropeanPatent No. 1086057 which describes the use of amorphous cast-ironplatelet particles in oil-well cements to provide added toughness andimpact resistance. However, there is a large density difference betweenthe particle and the base fluid—the cement slurry with which theparticles are mixed—so special care must be taken to prevent thesedimentation of the particles. When the particles are metallic, forinstance based on cast iron, the density difference is commonly around5000 kg/m³. Preventing the sedimentation of these fibres is usuallyensured by viscosifying the base fluid. The rheology of the base fluidis characterized by a minimum of two parameters, the high shear rateviscosity and the yield stress, which quantifies the low shear rateviscosity of the fluid. There are many drawbacks to viscosifying thebase fluid, for instance increase friction pressure drops when the fluidis pumped through tubulars, difficulties in mixing it and increased costdue to the use of viscosifying agents.

EP 621 247 describes the use of selected particle size distributions forsolid materials used in cementing slurries to provide slurries that arestable against settling or sedimentation.

EP 721 050 and U.S. Pat. No. 5,501,275 both describe fluids containing abase fluid, a first fibre or particle component and a second fibrecomponent, and relate to the use of fibres in the fluid to limit themovement of particulate materials in the fluid such as sand or proppantparticles

SUMMARY OF THE INVENTION

It is an object of the invention to provide a fibre-containing welltreatment fluid suitable for use in well cementing operations in whichthe settling of particles or fibres is inhibited.

In accordance with the present invention, there is provided a slurrycomprising a base fluid, first component comprising a fibrous orparticulate material and a second component comprising a fibrousmaterial, characterised in that the base fluid comprises a cementslurry, the first component is substantially more dense than the basefluid, and the second component has a density close to that of the basefluid and is relatively flexible.

The present invention addresses the problem of settling of the firstcomponent by providing a fluid containing a second fibre component withdiffering properties which can form a network in the base fluid whichtraps the first fibre component and prevents or hinders settling.

The first fibre component is typically a metallic material, such asamorphous cast iron, which can be present in the form of platelet-likestructures having an average length that is less than 10 mm. Such afibre is relatively short, dense and rigid.

The second fibre component is typically a glass, carbon or polymericmaterial in the form of long, flexible fibres or ribbons. Such materialstypically have a density close to that of the base fluid (cement) and anaverage fibre length in the range 5-35 mm. The fibres of the secondcomponent are preferably at least as long as, and thinner than, those ofthe first component.

The second fibre component is typically present in an amount of lessthan 10% by mass of the total amount of fibre in the fluid.

The cement slurry forming the base fluid can be any form of cementslurry that is suitable for well cementing operations, in particular ashear thinning slurry.

By adopting the approach of this invention, it is possible to maintainthe fibres in suspension without the need to increase the viscosity ofthe fluid with the inherent operational problems that can be causedthereby.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of examples and withreference to the accompanying drawings, in which:

FIG. 1 shows the experimental apparatus;

FIG. 2 show plots of pressure vs. time for runs A2 and A3;

FIGS. 3 a and 3 b show plots of sedimentation time vs. effectiveconcentrations C1 and C2 respectively; and

FIGS. 4 a and 4 b show photographs of sediment cake with Samples 1 and 5respectively.

DETAILED DESCRIPTION OF THE INVENTION

The present invention can be demonstrated by sedimentation experimentsperformed in a vertical tube T, with a diameter of 5 cm and a length of80 cm. The fibre concentration is determined by monitoring the pressuregradient using a Validyne differential pressure gauges ΔP1, as shown onFIG. 1. The differential pressure gauges ΔP1, is calibrated using acolumn of water. All measurements are expressed in terms of density, byconverting the pressure records according to the following calibrationdata.

Transducer ΔP1 ΔP2 ΔP3 Membrane nb and range 30-86 mbar 26-35 mbar 22-14mbar (1.25 psi) (0.5 psi) (0.2 psi) Sensitivity/calibration 2.0 V/mbar3.76 V/mbar 3.69 V/mbar slope Δp_(i) = ΔP_(i) | gΔH_(i) Eq. 1

The experimental procedure comprises the following steps:

The sedimentation tube is filled with water and all tubes purged of airbubbles. The equilibrium transducer output is recorded to be used asbaseline.

-   -   The experimental base fluid is mixed: 12 g of biozan (a        biopolymer) are added to 3 L of water and the solution is        stirred for 40 min to allow full hydration of the polymer, using        a paddle mixer. Antifoam and biocide are added. This base fluid        is chosen for its rheology and essentially inert behaviour        towards the fibres used in the experiments while corresponding        closely to the behaviour of cementing fluids without setting in        the manner of a cement.

The base fluid is tested to determine its rheology.

The dense fibres (SG Seva Fibraflex FF5E0 “Fibraflex”) are added to thebase fluid. When flexible fibres are used, 500 mL of base fluid arepoured in a Waring blender. The flexible fibres are dispersed byrotating at low speed. The rotation speed is adjusted in order to keepthe vortex. The suspension is then poured back in the main containerwhere Fibraflex fibres are added.

In the experiments, the suspension is either poured into thesedimentation tube or is pumped using a peristaltic pump to fill thetube from the bottom.

The pressure gradients are recorded on paper for a period of timeranging from 1 hour to overnight. The pressure decay is fitted with anexponential function allowing determination of the time constant.

Measurements are interpreted in terms of excess density with respect tothe base fluid. Knowing the density and concentration of FIBRAFLEXfibres (100 g/L), the theoretical density of the homogeneous suspensionis 1086 g/L. The excess density is therefore about 86 g/L.

The rheology of five batches of base fluid is monitored using a Fann 35,R1B1F1, to assess its reproducibility. The corresponding data and plotare shown below (Table 1). The rheology of the polymer solution isstable for more than one day.

TABLE 1 Sample A B C D E 300/200/100/60/30/6/3 RPM 25/21/17/15/12/8/727/23/19/17/13/9/8 27/22/18/15/13/9/8 27/23/19/17/14/10/926/22/18/15/13/9/8 Gel: 10 min/1 s/stir 12/9/8 14/10/9 12/9/8 12/10/9 —readings HB parameters: 0.67/0.437/1.94 0.97/0.392/2.0 0.346/0.54/3.10.82/0.401/3.05 0.583/0.45/2.94 K (Pa · s^(−n))/n/T₀ (Pa) Binghamparameters 13.3/5.8 12.4/7.03 15.3/5.75 13.3/6.75 14.3/5.85 PV (cP)/YP(Pa)

Six different flexible fibres (Samples 1-6) are studied. Theirproperties are gathered in Table 2 below.

Sample 1: short polyamide (nylon 6-6) fibres.

Sample 2: long polyamide fibres.

Sample 3: polypropylene ribbons, with a broad length distribution.

Sample 4: glass fibres with 40% water.

Sample 5: novoloid fibres with 20% water.

Sample 6: PET fibre. Its formulation includes a dispersant to enhanceits dispersability in water. When the fibres are cleaned with an organicsolvent, they become very difficult to disperse in water.

TABLE 2 Sample FIBRAFLEX 1 2 5 3 4 6 Material Cast iron polyamidepolyamide novoloid polypropylene fiberglass Polyester Shape plateletsrods rods rods platelets rods rods Length 5 mm 12 mm 19 mm 20 mm 12-35mm 12 mm 6 mm Diameter 0.8 mm 18 μm 18 μm 21 μm 0.8 mm 20 μm ~10 μm orwidth wide wide

The following suspension compositions are analysed, where theconcentrations are defined per liter of water.

TABLE 3 Reference and Sedimentation time (minutes), placement methodBase fluid Fibres mean fluctuation (%) A1 Biozan 4 g/L Fibraflex 100 g/L154 (0.31%) Poured from top Antifoam 1 g/L Sample 2: 2 g/L A2 Biozan 4g/L Fibraflex 100 g/L 4 (1.2%) Poured from top Antifoam 1 g/L Biocide 1g/L A3 Biozan 4 g/L Fibraflex 100 g/L 101 (0.35%) Poured from topAntifoam 1 g/L Sample 1: 2 g/L Biocide 1 g/L A4 Biozan 4 g/L Fibraflex100 g/L 134 (0.35%) Poured from top Antifoam 1 g/L Sample 1: 3 g/LBiocide 1 g/L A5 Biozan 4 g/L Fibraflex 100 g/L 544 (0.18%) Pumped frombottom Antifoam 1 g/L Sample 1: 4 g/L Biocide 1 g/L A6 Biozan 4 g/LFibraflex 100 g/L 52 (0.38%) Pumped from bottom Antifoam 1 g/L Sample 1:1 g/L Biocide 1 g/L A7 Biozan 4 g/L Fibraflex 100 g/L 571 (0.34%) Pumpedfrom bottom Antifoam 1 g/L Sample 5: 2 g/L Biocide 1 g/L A8 Biozan 4 g/LFibraflex 100 g/L 65 (0.21%) Pumped from bottom Antifoam 1 g/L Sample 4:2 g/L Biocide 1 g/L A9 Biozan 4 g/L Fibraflex 100 g/L 47 (0.62%) Pumpedfrom bottom Antifoam 1 g/L Sample 3: 2 g/L Biocide 1 g/L A10 Biozan 4g/L Fibraflex 100 g/L 100 (0.13%) Pumped from bottom Antifoam 1 g/LSample 5: 1 g/L Biocide 1 g/L A11 Biozan 4 g/L Fibraflex 100 g/L 2100(0.11%) Pumped from bottom Antifoam 1 g/L Sample 6: 2 g/L Biocide 1 g/LA12 Biozan 4 g/L Fibraflex 100 g/L 283 (0.15%) Pumped from bottomAntifoam 1 g/L Sample 6: 1 g/L Biocide 1 g/L

A first general observation is that the initial excess density (FIG. 2)is in quite good agreement with the theoretical value of 86 g/L. Anothergeneral observation is that large pressure fluctuations are observed forall experiments, starting at the beginning and lasting until no moreFIBRAFLEX fibres are left in suspension in between the pressure ports ofthe transducer. From visual observations, two phenomena may explainthese observations:

The FIBRAFLEX fibres tend to settle as large aggregates.

In a few cases, convection cells are clearly observed especially whenthe settling velocity is fast.

A measure of the amplitude of these fluctuations is provided (Table 3)as the mean square difference between the measurements and theexponential fit.

Results for runs A2 and A3 are shown in FIG. 2 and are typical.

The sedimentation times are plotted versus the concentration of flexiblefibres on FIGS. 3 a and 3 b. Logically, the sedimentation timeincreases—faster than linearly—when the concentration of flexible fibresincreases. Also, the more dispersed the fibres are, the strongest theeffect: Sample 6 are dispersed in individual fibres, Sample 5 areslightly more difficult to disperse while the polyamide andpolypropylene fibres are clearly hydrophobic and remain stuck together.In summary, the experimental data fall in three groups:

Very large effect, Sample 6;

Average effect: Sample 5, Sample 4, Sample 2, Sample 1;

Small effect: Sample 3.

In dilute suspensions, the relevant parameters that characterize thesettling of fibres are their hydrodynamic size and their effectiveconcentration calculated based on this size. If N is the number offibres per unit volume and L their length and d the fibre diameter, therelevant dimensionless numbers are:c ₁ =N(L/2)³c ₂ =N(L/2)²(d/2)c ₃=2πN(L/2)(d/2)²  Eq. 2c₁ represents the hydrodynamic volume, in dilute conditions. c₂ is thevolume of the disks whose diameter is equal to the fibre length: it isconsidered to be the relevant parameter in semi-dilute regime. c₃ is thematerial concentration.

For c₁<1, the regime is dilute; if c₁>1>c₂ this is a semi-dilute regimeand the suspension is considered to be concentrated when c₂>1. Table 4provides these effective concentrations, based on a concentration offlexible fibres of 2 g/L.

TABLE 4 Rods Ribbons Sample 4 Sample 5 Sample 2 Sample 1 Sample 6 Sample3 Fibraflex Cylinder diam./Width (μm) 20 21 18 18 10 800 1000 Thickness(μm) — — — — — 23 25 Length (mm) 12 20 19 12 6 12 to 35 5 Density (kg/L)2.5 1.27 1.1 1.1 1.37 0.9 7.2 Concentration (g/L) 2 2 2 2 2 2 100Nb/volume (m⁻³) 5.31E+07 5.68E+07 9.40E+07 1.49E+08 7.44E+08 1.01E+071.11E+08 Material conc., c₃ (vol %) 0.08% 0.16% 0.18% 0.18% 0.15% 0.22%1.39% Effective conc. c₁/c₂ 11/0.02 56/0.06 80/0.08 32/0.05 20/0.032/0.15 2/0.3

Clearly, the suspensions must be considered to be in the semi-diluteregime.

The fact that the fibres are intimately interpenetrating (c₁>>1)explains the suspension properties: they form a kind of a gel thatphysically hinders the settling of the heavy FIBRAFLEX particles. It ispossible to make sure that, when the FIBRAFLEX particles settle, they doso through the network of flexible fibres. The clear supernatant liquidat the top of the column still contains flexible fibres and the sedimentdeposited at the bottom of the tube contains a mixture of both fibres,clearly visible in FIGS. 4 a and 4 b. When flexible fibres are present,the cake of FIBRAFLEX particles is much less compact to a point thatsometimes it can be put back in flow with no external mixing.

Using its hydrodynamic volume and Stokes law, the settling velocity of asingle FIBRAFLEX particle would be around 0.5 mm/s, considering aneffective solution viscosity of 0.5 Pa·s (at 10 s⁻¹). This value is inagreement with the mean settling velocity of experiment A2 (no flexiblefibres used), v˜20 cm/4 nm=0.8 mm/s.

The exponential behaviour of all measured data corresponds to asedimentation regime governed by interactions between sedimentingparticles: for non-interacting particles, a linear behaviour should beobserved corresponding to the downward displacement of the upper front.This observation re-enforce the fact that the fibre suspension is in asemi-dilute regime.

The density fluctuations observed in almost all experiments mayoriginate from the difficulty in preparing homogeneous fibressuspensions, especially when the fibres are hydrophobic (Samples 2 and3). Also, the significant length of the fibres can lead to wrappingaround any rotating instrument, possibly resulting in kind of “balls ofyarn”. There is some correlation between the amplitude of thesefluctuations and the nature of the fibres: the amplitude decreases forlarge fibre concentration and hydrophilically treated materials (Samples4 and 5).

In practical applications, the two fibre components are added to theother components of the cement slurry in the usual manner. The preciseamounts of fibres to be added can be determined by simpleexperimentation according to the required performance of the slurry.Once mixed, the slurry including the fibres is pumped into the well inthe normal manner.

1. A cement slurry, comprising a base fluid including cement, a firstfibrous component made of metallic fibres and a second fibrous componentmade of glass, carbon or polymeric fibres having a density close to thatof the base fluid, said second fibrous component present at aconcentration of less than 10% by mass of the total fibrous content ofthe fluid.
 2. The cement slurry of claim 1, wherein the metallic fibrescomprise amorphous cast iron.
 3. The cement slurry of claim 2, whereinthe metallic fibres are flat, plate-like structures having an averagelength less than 10 mm.
 4. The cement slurry of claim 1 wherein thesecond fibrous component has a length ranging from 5 to 35 mm.
 5. Thecement slurry of claim 1, wherein the second fibrous component isselected from the list consisting of glass, polyamide, novoloid,polypropylene and polyester fibres.
 6. The cement slurry of claim 5wherein the second fibrous component has a length ranging from 5 to 35mm.
 7. The cement slurry of claim 1, wherein the base fluid exhibitsshear-thinning behaviour.
 8. A method of treating a well, comprisingpumping into the well a cement slurry as claimed in claim
 7. 9. A methodof treating a well, comprising pumping into the well a cement slurry asclaimed in claim 1.